The invention relates to improvements in methods and machines for grinding internal threads, especially ball threads.
Presently known machines for grinding internal threads in nuts and other types of workpieces normally employ grinding wheels which are made of corundum. Such grinding wheels are dressed, almost invariably, with radius dressing tools employing a single-point diamond. If, in some rare instances, the grinding wheel is dressed with a diamond profiling roll, the inclination of the axis of the roll with reference to the axis of the grinding wheel must conform to the lead angle. In the course of the grinding operation, the axes of the workpiece and grinding wheel also make an angle which equals the lead angle of the thread to be ground. Thus, when the working surface of the grinding wheel is being dressed by a diamond profiling roll, the axis of the dressing roll is parallel to the axis of the grinding wheel. This ensures that the profile of the roll can be accurately transferred onto the grinding wheel. In other words, the grinding angle (between the axes of the workpiece and grinding wheel during grinding) equals the lead angle (of the thread to be ground), and the dressing angle (between the axes of the grinding wheel and the rotary diamond profiling roll) equals zero.
The utilization of more effective cubic boron nitride (CBN) grinding wheels and their dressing with diamond profiling rolls is unwarranted for economical reasons or is plain impossible in connection with internal grinding in accordance with the above outlined conventional method. This is due to the fact that the arbor of the grinding spindle (the arbor actually carriers the grinding wheel) is too weak for transmission of forces which must be applied for grinding with a CBN grinding wheel and which must be applied to dress a CBN grinding wheel with a diamond profiling roll. The maximum diameter of the grinding spindle arbor is a function of the grinding angle and of the ratio of length of the thread to the thread core diameter. If the internal thread is long and thread core diameter is small, the diameter of the arbor of the grinding spindle is necessarily small and the arbor is so weak that it cannot transmit and/or standard forces which are required for efficient operation of a CBN grinding wheel and which developed when a CBN grinding wheel is dressed with a diamond profiling roll. It is further to be borne in mind that the inclination of the arbor is dependent upon the required grinding angle and its plane of contact with the internal thread is off center so that the arbor is even closer to the innermost portion of the internal thread, i.e., to the thread core diameter. An arbor whose diameter is small exhibits a number of serious drawbacks. Thus, the arbor has a pronounced tendency to vibrate so that it cannot be subjected to large radially oriented stresses. The output is unsatisfactory because the critical rotational speed of a small-diameter arbor is very low. Operation with CBN grinding wheels is not economical because such grinding wheels cannot be used to capacity due to the weakness of a small-diameter arbor, and the output of a corundum grinding wheel (particularly in comparison with that of a fully effective CBN grinding wheel) is equally unsatisfactory. As mentioned above, a relatively weak arbor cannot withstand stresses which develop during dressing of a CBN grinding wheel with a diamond profiling roll. This renders it necessary to increase the number of grinding machines and have each machine operate with a corundum grinding wheel at a relatively low output.